Prop Question

[PGtoy]

I changed my prop the old one was damaged<br />Old: 15 X 17 3 blade WOT 4600 - 4800 RPM<br />Speed 3800 was 24MPH<br />New: 15 X 16 4 blade WOT 4100 - 4200 RPM<br />Speed 3400 is 28 MPH<br />WOT by the book is 4400 - 4600 RPM <br />5.7 merc 2 bl carb Alpha 1 gen 2 drive<br />Will running the 4 blade prop hurt anything or should I look at a 15 X 19 3 blade prop

 

[QC]

Re: Prop Question

What was the load like when you tested it. Light? Heavy? or typical? The way it is now is not good unlesws you were running the heaviest load you ever have before.<br /><br />That old prop seems like it was slipping a LOT! Could it have been worked on before? i.e. less pitch than noted? Going to the 19 is the wrong direction to fix what appears to be the issue. The fact that your 17 turned more RPM than the 16 tells me something is way wrong with the info. Should be the exact opposite.

 

[PGtoy]

Re: Prop Question

The 17 is a 3 blade the 16 is a 4 blade. The load on the boat was a full fuel 75gal full water 20 gal all the gear and two people ( typical )

 

[QC]

Re: Prop Question

Something is still wrong with the 17 inch info. It was either reworked, incorrectly marked or damaged when you got those numbers. This assumes that the info on the 16 is crorrect. the 3 blade vs. 4 blade will have some impact, but the pitch is still the more important number. You should read the prop threads in the Engine FAQ section. With what you have listed here I would go to a 14 inch pitch four blade, but that assumes that I throw out the 17 inch info. It just doesn't jive . . .

 

[Mahoney]

Re: Prop Question

I agree, your pre and post prop numbers look backwards. Even if the props were excactly the same pitch and you went to 4 blades from 3, all things being equal, you could expect to actually lose 2-3 MPH.<br /><br />UNless these are on totally different boats, there is no way no how, going down in pitch will increase top end and decrease WOT. <br /><br />Must have been a mismarked prop

 

[PGtoy]

Re: Prop Question

The three blade could have been miss marked I had a Marina put it on when I bought the boat.<br />I just tried the four blade prop I bought new 15 X 16 pitch. The boat went 32MPH with a max WOT of 4100. The rating is for 4200 to 4600. If I keep the four blade on will this hurt the drive or engine. I am thinking of a 3 blade again 14 1/2 X 19 pitch<br />All the testing is done with a 2452 Bayliner fully loaded.

 

[QC]

Re: Prop Question

Definitely DO NOT do the 19. That is a big boat, no way it swings the 19. You are still going the wrong way. Based on what you are reporting, you need to go down to a 14" pitch. What you have on there now is not good. You will be much happier and so will your boat with a prop that runs in the recommended RPM range at WOT.

 

[PGtoy]

Re: Prop Question

So The best I can do is go back to a 3 blade 15 X17 pich that did run at WOT 4600. This prop thing can really make you crazy. Since I damaged the original 15 X 17 3 blade I have been trying how to get a little more speed whihout damage to the drive or engine. Thanks for all the help

 

[QC]

Re: Prop Question

Yes, my guess it is no longer a 17 and has been monkeyed with to get it right for that boat. You need two on board, so I would look for another 4 blade in a lower pitch, again, maybe as low as 14. Have a good season.

 

[achris]

Re: Prop Question

Merc only do the 4 blade ali props in 16, 18 and 20". I was running a 3 blade 21" pulling 40 knots at 4900rpm, changed to the smoother 4 blade (20") lost 3 knots and about 250rpm. You could try a 4 blade S/S prop. They are very nice.<br /><br />Chris..........